(RawStory) – Senior Bush administration officials sternly cautioned the 9/11 Commission against probing too deeply into the terrorist attacks of September 11, 2001, according to a document recently obtained by the ACLU.

The notification came in a letter dated January 6, 2004, addressed by Attorney General John Ashcroft, Defense Secretary Donald H. Rumsfeld and CIA Director George J. Tenet. The ACLU described it as a fax sent by David Addington, then-counsel to former vice president Dick Cheney.

In the message, the officials denied the bipartisan commission’s request to question terrorist detainees, informing its two senior-most members that doing so would “cross” a “line” and obstruct the administration’s ability to protect the nation.

“In response to the Commission’s expansive requests for access to secrets, the executive branch has provided such access in full cooperation,” the letter read. “There is, however, a line that the Commission should not cross — the line separating the Commission’s proper inquiry into the September 11, 2001 attacks from interference with the Government’s ability to safeguard the national security, including protection of Americans from future terrorist attacks.”

The 9/11 Commission, officially called the National Commission on Terrorist Attacks Upon the United States, was formed by President Bush in November of 2002 “to prepare a full and complete account of the circumstances surrounding the September 11, 2001 terrorist attacks” and to offer recommendations for preventing future attacks.

“The Commission staff’s proposed participation in questioning of detainees would cross that line,” the letter continued. “As the officers of the United States responsible for the law enforcement, defense and intelligence functions of the Government, we urge your Commission not to further pursue the proposed request to participate in the questioning of detainees.”

FireDogLake’s Marcy Wheeler speculates that this was an attempt by the Bush administration to ensure that its torture of certain detainees, which has since been widely documented, remained secret.

“[W]hoever made these annotations appears to have been most worried that Commission staff members could make independent judgments about the detainees and the interrogations,” Wheeler wrote on her blog. The official “didn’t want anyone to independently evaluate the interrogations conducted in the torture program.”

Eventually, the commission’s co-chairs harshly criticized the administration for having purportedly “destroyed” tapes of its interrogations with terror suspects, as Raw Story reported last year.

9/11 Commission members Thomas Kean and Lee H. Hamilton wrote that although US President George W. Bush had ordered all executive branch agencies to cooperate with the probe, “recent revelations that the CIA destroyed videotaped interrogations of Qaeda operatives leads us to conclude that the agency failed to respond to our lawful requests for information about the 9/11 plot.”

“Those who knew about those videotapes — and did not tell us about them — obstructed our investigation.”

They continued: “There could have been absolutely no doubt in the mind of anyone at the CIA — or the White House — of the commission’s interest in any and all information related to Qaeda detainees involved in the 9/11 plot.

“Yet no one in the administration ever told the commission of the existence of videotapes of detainee interrogations,” Kean and Hamilton wrote.